The scientific motivation of this project is to investigate the interplay between Active Galactic Nuclei and their host galaxies. The radio ejecta from AGN can severely impact on the way that stars are formed in galaxies - they can either heat the gas and so prevent it from collapsing into stars, or they can compress clouds of gas, thereby triggering star formation.
It is therefore necessary to determine which galaxies do have radio-active AGN. Whilst there are several diagnostics, a relatively easy and direct way is a detection in a VLBI observation. The reason is that the emission needed to make a detection must come from a very small volume in the target object, because the resolution is so high in VLBI observations. This is typically expressed as a brightness temperature, which is the temperature a black body would need to have to produce the same flux density. The brightness temperatures required for VLBI detections are of order 106K, and it is commonly accepted that nothing becomes so hot on the outside. Therefore one concludes that these sources are non-thermal and produce synchrotron emission. A sample of radio sources which have been detected in VLBI observations therefore constitutes a clean sample of radio-active AGN.
Unfortunately, a non-detection in a VLBI observations does not imply that there is no AGN, not even a radio-active AGN. The emission can be below the detection limits of the observations or the radio emission can be switched off temporarily (AGN are known to have duty cycles).
To obtain a sufficiently large sample of galaxies we have decided to observe the radio sources in the COSMOS field. It not only has sensitive VLA data which are needed to position the VLBI phase centres, it also comes with an enormous amount of ancillary data in the optical, infrared, and X-ray.
This project is the culmination of a lot of preparatory work, including the efforts of a large number of individuals. Its roots go back to 2006, when the idea of carrying out such a survey was first conceived. Publications related to this project are: